1. Field of the Invention
The present invention relates to an organic field effect transistor and a method of manufacturing the same and more particularly, to an organic field effect transistor with a long life span by employing a channel made of an organic semiconductor material.
2. Description of the Related Art
An organic field effect transistor using an organic semiconductor has been researched as a substitute for a transistor made of an inorganic material, such as silicone, which has been widely used, for example, an amorphous silicone thin film transistor used for switching control in a liquid crystal display device, and recently, a device with performance as good as the amorphous silicone thin film transistor has been suggested.
The organic field effect transistor has several characteristics when compared to a field effect transistor (hereinafter, referred to as an inorganic field effect transistor) now widely in use based on a silicone semiconductor.
For example, since an organic field effect transistor can use a relatively flexible material, a flexible circuit can be manufactured and the organic field effect transistor is useful for a flexible device and so like.
In addition, since a vacuum process is not required and elements can be formed by a printing method, the organic field effect transistor can be manufactured through a relatively simple process as compared to the inorganic field effect transistor, which investment for building a fabrication facility can be saved. In addition, the organic field effect transistor has an advantage in that it weighs light compared to the inorganic field effect transistor.
Since the organic field effect transistor has such good characteristics, comprehensive research on the organic field effect transistor is now under way. For example, the structure has been suggested in which a gate insulating layer is formed of a metal oxide or a metal nitride to reduce a gate voltage and a leakage current (for example, see Japanese Patent Laid-Open No. 8-228034).
Another structure has been suggested employing as a channel an organic semiconductor film made of a first p-typed organic material layer and a second n-typed organic material layer contacting with the first p-typed organic material layer for the purpose of improving an ON/OFF ratio of current between a source and a drain (for example, see Japanese Patent Laid-Open No. 2003-303970).
However, even in the above-mentioned organic field effect transistors, the ON/OFF ratio of current between the source and the drain or improvement of a gate insulating film and characteristic stability that is a condition to be met for practical use are not satisfactory.
As factors causing the instability in characteristics of the organic field effect transistor, various factors including decomposition or deterioration of a material itself due to power supply, physical change such as peeling off on the interface between the organic semiconductor layer and the electrode, and deterioration due to moisture or oxygen permeated from the outside are considered, but the exact factor has not been clearly discovered until now.
The current between a source and a drain with respect to a gate voltage (hereinafter, referred to as FET characteristics) and the ratio of current between the source and the drain at the time when a predetermined amount of voltage is applied into a gate electrode (that is, at the time of an ON voltage) against that at the time when no voltage is applied into the gate electrode (that is, at the time of an OFF voltage) (hereinafter, referred to as an ON/OFF ratio) may typical characteristics of the field effect transistor.
When the element characteristics are unstable, the FET characteristics or ON/OFF ratio are gradually changed from the time when the element is fabricated and are deteriorated in proportion to multiplication of power supply time and amount of power. Even in case in which no power is supplied into the element, its element characteristics gradually deteriorate due to oxygen or moisture in the atmosphere.
Therefore, the deterioration of element characteristics makes it difficult for the organic field effect transistor to be in practical use. As a result, improvement is required.